Metallization of optical fibers

ABSTRACT

A method is disclosed for the metallization of optical fibers with improved adhesion while using a shorter process cycle.

FIELD OF THE INVENTION

[0001] This invention relates to a method for plating metal on optical fibers.

BACKGROUND OF THE INVENTION

[0002] The technology of fiber optics is gaining increasing momentum in modern communication systems. In order to achieve solderability that is dictated by a host of applications, optical fibers need to be coated with metal. Sputtering, electroless or electrolytic plating of nickel , usually followed by gold, are the most frequently practiced metallization methods.

[0003] U.S. Pat. Nos. 5,380,559 and 6,251,252 are indicative of the prior art, based on multiple step sensitization using aq. solutions of stannous chloride and or fluoride, followed by aqueous solutions of palladium chloride. Such sensitization methods can be prone to incomplete metal coverage on the fiber, may lead to inadequate metal-to- fiber adhesion, and result in long process cycles. It is noted for example, that U.S. Pat. No. 5,380,559 requires “sensitization” of the optical fiber to take place in oxygen-free solutions, a restriction that makes the process cumbersome and not very user-friendly.

[0004] The prior art also suffers from narrow process window, as reflected in U.S. Pat. No. 6,187,378 B1, which describes an intricate, potentially restrictive and costprohibitive automated system, designed to keep stringently constant the rate of entry and withdrawal of the fibers to be plated, as they progress through the various process solutions, in the quest of achieving consistent results.

SUMMARY OF THE INVENTION

[0005] It is an object of this invention to minimize the shortcomings of the prior art by providing a method for depositing a metal layer on optical fibers, where said layer is complete and without “pinhole voids” (microscopic bare spots with no metal), is smooth, and devoid of imperfections such as graininess, roughness, bumps, “pimples”, etc. It is a further object of the invention to provide a method that offers improved metal-to-fiber adhesion, and is using a shorter and more economical production cycle than the prior art. Enabling as short a production process as possible, is especially important for metallizing optical fibers, where one often deals with a multitude fibers in the form of bundles.

[0006] It is thus a salient object of the invention to replace multiple-step sensitization with a one-step, single composition, to which fluorides can be added, as appropriate. In the prior art cited above, fluorides are placed in one or more sensitization steps preceding plating. This invention implements the beneficial effect of fluorides by offering several potential fluoride- bearing steps, thus affording greater flexibility.

[0007] Thus, the method of the invention for metallizing a silica surface of an optical fiber, comprises the steps of:

[0008] (i) contacting said silica surface with an aqueous colloidal composition comprising stannous chloride, palladium chloride and hydrochloric acid; and

[0009] (ii) depositing at least one metal layer on the surface obtained in (i).

[0010] According to a preferred embodiment, the method of the invention comprises the following steps:

[0011] (a) cleansing the surface of the exposed optical fiber, so as to eliminate contaminants from the surface of the optical fiber;

[0012] (b) contacting the optical fiber with a single sensitizing composition comprising stannous chloride, palladium chloride and hydrochloric acid; where the sensitizing solution may optionally contain fluorides;

[0013] (c) contacting the fiber obtained in step (b) with an accelerating solution;

[0014] (d) exposing the optical fiber obtained in step (c) to at least one electroless metal step and/or to at least one electroplating metal step to deposit at least one metal layer, said metal being selected from Ni, Cu, Ni/Cu, Ni/Co, Ni/Pd and combinations thereof, and

[0015] (e) plating a top metal layer such metal layer comprising a metal selected from Au, Pd, Ni/Pd and Pt.

[0016] Usually, the contaminants removed in step (a) are organic contaminants.

[0017] As mentioned above, the shorter process cycle and improved metal-to-fiber adhesion are principally based on a single-step sensitization technique, involving Sn/Pd/HCl, first disclosed in U.S. Pat. Nos. 3,011,920 and 3,532,518, and discussed mechanistically and operationally in the publication entitled “An Auger Spectroscopic Study of the Catalysis Process, Plating and Surface Finishing, October 1961, pp. 71-73”. Such Sn/Pd/HCl compositions are generally referred to in the trade interchangeably as activators, catalyst, sensitizers, etc. The single-step sensitization thus replaces the multiple sensitization steps of the prior art patents cited previously, that are based on individual solutions of SnCl₂, SnF₂ and PdCl₂.

[0018] The metallization in step (d) of the process of the invention may be carried out by either electroless or electrolytic techniques where, as mentioned earlier, the metal or metal alloy to be plated is selected from Ni, Cu, Ni/Cu, Ni/Co, Ni/Pd and combinations thereof. In cases where electroplating replaces electroless plating, electrolytic barrel plating is an option.

[0019] Compositions and methods for depositing electroless nickel, both acid and alkaline types, are well documented in the literature, and need not be dealt with here.

[0020] An optional embodiment of the present invention proposes heating the optical fiber after being completely covered with the initial metal layer, for the purpose of maximizing the metal/fiber interfacial bond. Such heating step can be implemented in many ways, via oven, hot gas, steam, etc., and will take place preferably, though not limitingly, prior to applying additional metal to the desired thickness. Heating the metal-plated fiber additionally or solely, after it received the desired metal thickness, as opposed to heating after only depositing a thin initial metal layer, may in certain special instances be more desirable as it will also, in addition to enhancing metal-to-fiber bonding, beneficially release at the same time the internal stresses often contained in some metal deposits.

[0021] The invention also foresees use of multiple metal films, whether deposited electrolessly or electrolytically, prior to the top metal layer which is typically gold, in order to offer greater flexibility to structurally strengthen the metal composite and, at the same time, maximize adhesion at the critical fiber/metal interface.

[0022] A still further embodiment of this invention envisions electroplating of the optical fiber without electroless plating altogether, a potentially revolutionary contribution to plated metal fibers. Direct electroplating of the fiber without an intermediate electroless metal can be achieved after strong, energetic sensitization in the Sn/Pd composition, or via other techniques that impart sufficient, albeit spare, electrical surface-conductivity for electroplating the fiber. Compositions and methods that enable electroplating on non-conductors without first resorting to eletroless deposition, can be found in U.S. Pat. No. 4,619,741, WO 0036189 (EP 1157149), with the latter proposing double-dipping in a Sn/Pd composition for improved conductivity. In the case of direct electroplating silica-based glass fibers without electroless, this patent envisions creating an adsorptive surface topography via aggressive “frosting” of the fiber surface using fluorides and the like, in order to reinforce surface adsorption of the electrically “conducivating” substance such as, for example, colloidal Sn/Pn.

DETAILED DESCRIPTION OF THE INVENTION

[0023] The process of this invention can be carried out as follows:

[0024] In the first step, the surface of the fiber needs to be cleaned (either mechanical or chemical cleaning) of residues, especially organic, that are usually left behind on the fiber following stripping. Contrary to the prior art (for example U.S. Pat. No. 6,251,252 that teaches treating the bare fiber in methylene chloride, a somewhat problematic step, both environmentally and ecologically), the present invention avoids/ minimizes the use of solvents in preparing the fiber for metal deposition. Thus, surfactant-based cleaners desirably containing fluorides are preferable used in the process of the invention. Alternative cleaning compositions are those comprising an aqueous permanganate solution, acid or alkaline, containing a fluoride such as ammonium bifluoride, and a surfactant, preferably cationic. Indeed, cationic surfactants will facilitate adsorption of the negatively charged Sn/Pd colloids, by imparting a positive charge to the fiber surface.

[0025] In practicing the invention, one skilled in the art will need to optimize cleaning conditions, i.e. time/temperature concentration/agitation, etc., to suit a given condition such as for example, type and tenacity of the soil residue on the surface of the fiber. As an indication only, the patent considers using potassium or sodium permanganate at about 10 g/l, fluorides at 5-10 g/l, a cationic surfactant such as Catanac also known as Cyastat SP and available from American Cyanamide, at concentrations of about 100-200 ppm. Again, such cleaning compositions can be acid or alkaline, and may occasionally require use of elevated temperatures to insure freedom from surface residues prior to sensitization. Pennanganates can often be replaced, if desired, by other oxidizers such as chromic acid, peroxides, persulfates and the like. Also, surface cleaning of the fiber to be metallized can additionally be enhanced by agitation, as exemplified in U.S. Pat. No. 6,178,974 B1.

[0026] It is recommended that the cleaned fiber surface be tested for cleanliness using the “water-break” test, as known in the industry of surface finishing. A helpful description and definition of the “water break” (WB) test can be found in Cleaning and Preparation of Metals, Plating and Metal Finishing, pp.1039-1045, September 1959.

[0027] Following cleaning, the fiber undergoes water -rinsing, done by immersion, spray, or the like. Unless specified differently, water-rinsing is desired in-between each process step. Also, throughout the Examples described below, vigorous agitation of the substrate to be plated was applied using magnetic stirring of the workpiece.

[0028] Next, the fiber is contacted with the composition of stannous chloride, palladium chloride and hydrochloric acid, for the purpose of catalyzing the fiber surface for electroless metal deposition, or in order to impart minimal electrical conductivity for electroplating the desired metal, without electroless deposition prior to it. Also, in the practice of the invention, one has the choice between utilizing commercial products and operating instructions offered by vendors, or by manufacturing/mixing working-compositions following teachings of the patents cited previously, or using other recipes reported in the literature. In an effort to achieve smooth, as opposed to rough metal layers, efficient filtration of the Sn/Pd/HCl composition is indicated. Filtration will be facilitated by preparing compositions with fine/small colloid particles, favored by the use of low Pd concentration, sometimes in the range of about 5-20 ppm, which is below vendors' ordinary recommendations. Effectiveness of the sensitiser or catalyst can be,accentuated by elevated operating temperatures, as high as 50-60 deg° C., while insuring stability of the catalyst against decomposition by maintaining high Sn⁺⁺/Pd ratios, in the range of 100/1, or considerably higher.

[0029] Subsequent to Sn/Pd activation, the fiber is “accelerated” by exposure to an acid or alkaline aqueous solution. The term “acceleration” as described above and discussed in the references cited previously, refers to the chemical treatment of the catalyzed surface after activation. The term has been widely adopted by the prior art Benefits of acceleration are believed (though not convincingly supported by scientific evidence), to result from removal of “excess” tin in the tin/palladium colloidal particles covering the surface to be plated. Though acceleration is normally recommended and almost universally practiced, it can optionally be skipped in this invention for functional benefits, as well as in order to shorten the overall process. In such instances, acceleration as described above, can be replaced with copious water rinsing, the water being at ambient temperature, or preferably heated to well above ambient temperatures, potentially up to boiling. The invention also foresees acceleration in aqueous solutions of fluorides, such as for example ammonium bifluoride, with still further potential benefits on adhesion, by virtue of ameliorating/“frosting” the surface topography of the silica fiber prior to its entry in the subsequent electroless and/or electrolytic bath.

[0030] In practicing electroless deposition, the prior art offers a vast array of formulas, compositions, methods, to choose from. It is incumbent on the person skilled in the art to select, adapt, optimize, a chemistry that is best suited for given optical fibers, with their unique set of problems/requirements, especially as they relate to interfacial metal-to-fiber adhesion. When opting for electroless nickel as the first layer, a preferred embodiment of this invention preferably calls for a first electroless nickel operated at ambient temperature, whether using compositions that are alkaline (ammoniacal) predominantly maintained at a pH of about 9, or the low pH type, normally run in the pH range of about 4-6. Indeed, lower temperatures lead in many instances to improved adhesion occasioned by slow, more “orderly” initial nucleation, as opposed to high temperature baths with their rapid and sometimes disorderly nickel deposition. After achieving initial nickel coverage which can be a fraction of a micron thick, the fiber can be transferred, often without a water-rinse, to an electroless nickel bath at higher operating temperature in the range of 70-90 deg° C.:, with its higher deposition rate, until completion of the desired thickness. Still, in some special applications, one may choose electroless nickel reduced by boranes, such as borohydrides, dimethylamine borane, etc., due to their improved solderability, relatively low temperature operation, and increased catalytic behavior that require minimal Pd surface concentration for initiating, triggering electroless deposition.

[0031] A still further embodiment of this invention utilizes electroless copper as the very initial layer following sensitization. In such case, preference is given to hypophposphite-reduced copper compositions and methods exemplified in U.S. Pat. Nos. 4,279,948, 4,265,943 and 4,209,331, as opposed to formaldehyde electroless copper chemistries, for both environmental and improved functional considerations of the copper deposit, such as smoothness, specularity, improved adhesion to the fiber, and others. Also, hypophosphite-reduced coppers have unusually high time/temperature stability, vs. formaldehyde-types that are thermodynamically unstable. The hypophosphite-reduced copper deposit which is typically about 0.1 micron thick or even less, is co-deposited with some phosphorus, forming a Ni—P compound, and is typically followed by electroless or electrolytic nickel to a desired thickness. It is puzzling that in the prior art of plating of optical fibers, electroless copper is somewhat underemphasized. A still additional embodiment of the invention contemplates deposition of electroless Ni/Cu as disclosed in U.S. Pat. No. 3,764,352 in lieu of, or in addition to other electroless deposits.

[0032] For embodiments of this invention that directly electroplate nickel or copper over the fiber, and thus obviate the use of electroless copper altogether, there is a great reservoir of electrolytic processes and compositions in addition to patents referenced previously, known as “Direct Plating”or “Direct Metallization” generated lately in the field of Printed Circuit nterconnections and Throughhole Plating. Adaptation to direct electroplating of fibers is affordable by one skilled in the art.

[0033] In describing the invention, it has been shown that it addresses some of the significant shortcomings of the prior art of metallizing fibers, namely, inconsistent adhesion, narrow and somewhat protracted process cycle, a greater choice of electroless or electrolytic methods of metal deposition, and improved economy.

[0034] The selection and method of deposition of the final, or top metal layer such as gold, is outside the scope of the invention. It is however a further potential advantage/benefit of this invention, that by plating alloys such as Ni/Cu, or Ni/Pd, or Ni/B, one can insure/ preserve solderability without the need for gold or similarly expensive finishes.

[0035] Finally, while this invention prefers a single-step Sn—Pd—HCl catalyst sensitizer some, if not necessarily all of the advantages outlined previously are achievable in following the concept and teachings of the patent, even when using multiple-step sensitizers of the prior art. The latter are therefore within the realm/ domain of the broad aspects of the invention.

EXAMPLES Example 1

[0036] A 3″×3″ silica glass cloth, the type used in manufacturing “prepreg” for glass-epoxy laminates Printed Circuits, was metallized as follows:

[0037] 1. Immerse at 30 deg. C. for 5 min. in aqueous solution containing 10 g/l of ammonium bifluoride and 200 ppm of a cationic surfactant commercially known as Catanac or Cyastat SP, available from American Cyanamid;

[0038] 2.Rinse copiously with water;

[0039] 3. Immerse at 30 deg. C., for 5 min. in 0.5% v/vo of Mactivate 10 * (a composition comprising stannous chloride, palladium chloride and HCl, to which 5% v/vo HCl was added, and in which 5 g/l of ammonium bifluoride was dissolved;

[0040] 4. Water rinse 5.Immerse for 5 min. in 10% v/vo of an acidic accelerator 9071*

[0041] 6. Double water rinse

[0042] 7.Electrolessly plate nickel by immersing at 30 deg. C., in Metex 9340 * pH about 9, for 5 min.

[0043] 8. Immerse in Metex 8030 * electroless nickel, pH about 4.7, at about 80 deg. C., for 10 min.

[0044] 9. Water rinse, dry.

[0045] Following step #2, the fiber cloth sustained a continuous water film, i.e. passed the WB test.

[0046] In step #7, it was observed that electroless nickel coverage was completed after only 1 min. in the solution.

[0047] Total nickel thickness following step 9 was about 2.5 microns. The surface of the nickel was smooth, with no bumps, as observed under a microscope.

[0048] Adhesion was measured by the somewhat crude method of creasing the cloth 10 times, then applying scotch tape to the creased area, removing the scotch tape, and inspect the tape for any nickel adhering to it. None was observed.

[0049] In the process, no water rinse was used between the alkaline and acidic nickel (i.e. between steps #7 and #8), without visibly detrimental effect. Still, a water rinse between the two nickel solutions may at times be optionally desirable.

[0050] Also, in this Example it was noted after step #4 that the otherwise clear, colorless fibers of the glass cloth emerged from the water rinse with a noticeable, sustained brownish color, presumably due to adsorption of the brown colloids from the Sn/Pd colloidal composition. This phenomenon can be accentuated by the use of more concentrated Sn/Pd composition, and is of potential assistance

[0051] outside of plating, for the purpose of coloring and/or coating the fiber glass core with brown tin-palladium particles for possible optical or other beneficial surface effects.

[0052] Note: compositions denoted by (*) available from MacDermid Israel Ltd., under license of MacDermid Inc.

Example 2

[0053] Same as Example 1, except that step #2 was omitted. Total nickel thickness was about 2.4 microns. Appearance of the nickel layer as well as adhesion were comparable to Example 1. Still, the “double-nickel” method as in Example 1, is intuitively deemed better for demanding functional requirements, and would likely be confirmed under more severe testing.

Example 3

[0054] Same as Example 1, except:

[0055] Steps #1, 2, 4,5, were omitted.

[0056] The catalyst composition used in step #3 is as follows:

[0057] 1000 cc DI water

[0058] 2 cc/l of Mactivate 10 *, corresponding to approximately 20 ppm of Pd, which is about 20% of the 100 ppm recommended by the supplier.

[0059] 100 g/l NaCl

[0060] 25 g/l of SnCl2

[0061] The temperature of the above catalyst composition was kept at 50-55° C., and the fiber cloth was immersed in it for 5 min, with vigorous agitation, followed by copious water rinsing as in step #6, Example 1.

[0062] Initial nickel deposition in the first bath (Metex 9340 *, step #7 of Example 1) was somewhat slower, with coverage completed well within 5 min. After 1 min. in Metex 8030 * as in Example 1, water rinse and dry, the total thickness was 2.5 microns, the smoothness of the nickel surface and adhesion were the same as in Example 1.

[0063] The tin/palladium composition used in this example was kept at 50° C. for over four weeks, and surprisingly /unexpectedly suffered no noticeable decomposition.

[0064] Example 3 demonstrates feasibility of a much needed “ultra short” process cycle, coupled with very low concentration of Pd, economically attractive and made possible by the invention 

1. A process for metallizing a silica surface of an optical fiber, said process comprising: (i) contacting said silica surface with an aqueous colloidal composition comprising stannous chloride, palladium chloride and hydrochloric acid; and (ii) depositing at least one metal layer on the surface obtained in (i).
 2. The process according to claim 1, wherein said metal layer is deposited electrolessly.
 3. The process according to claim 1, wherein the metal layer is deposited electrolytically.
 4. The process according to claim 2, wherein the metal layer is nickel.
 5. The process according to claim 1, wherein the metal layer is copper.
 6. The process according to claim 5, wherein the metal layer is deposited electrolessly.
 7. The process according to claim 1, wherein the formation of the metal layer in step (ii) is followed by deposition of at least one more metal layer.
 8. The process according to claim 1, wherein the metal layer is Ni—B.
 9. The process according to claim 1, wherein the surface obtained in (i) is contacted with an aqueous accelerating solution.
 10. The process according to claim 8, wherein the accelerating solution comprises ammonium bifluoride dissolved in water.
 11. A colloidal catalyst composition for use in plating optical fibers, comprising inorganic fluorides in addition to water, stannous chloride palladium chloride and hydrochloric acid.
 12. A method for metallizing a silica surface of an optical fiber comprising the following steps: (a) cleaning the surface of the exposed optical fiber, so as to eliminate contaminants from the surface of the optical fiber; (b) contacting the optical fiber with a single sensitizing composition comprising stannous chloride, palladium chloride and hydrochloric acid; where the sensitizing solution may optionally contain fluorides; (c) contacting the fiber obtained in step (b) with an accelerating solution; (d) exposing the optical fiber obtained in step (c) to at least one electroless metal step and/or to at least one electroplating metal step to deposit at least one metal layer, said metal being selected from Ni, Cu, Ni/Cu, Ni/Co, Ni/Pd and combinations thereof, and (e) plating a top metal layer, such metal layer comprising a metal or metal alloy selected from Au, Pd, Ni/Pd and Pt.
 13. The method according to claim 12, where the metallization in step (d) is carried out by electroless deposition.
 14. The method according to claim 12, where the metalization in step (d) is carried out by electroplating.
 15. The method according to claim 12, further comprising after step (d) and before step (e) a step of heating the metal-plated optical fiber.
 16. Optical fiber metallized by the method of claim
 1. 17. Optical fiber metallized by the method of claim
 12. 18. A method for depositing a tin- palladium layer on a silica surface of an optical fiber, said method comprising contacting said silica surface with an aqueous colloidal composition comprising stannous chloride, palladium chloride and hydrochloric acid, so as to obtain a silica surface coated with a layer comprising tin-palladium particles.
 19. The method of claim 18, further comprising rinsing with water said coated silica surface and drying so as to obtain a dry layer comprising tin-palladium particles.
 20. The method of claim 18 or 19, wherein said layer is colored.
 21. Optical fiber processed by method of claim
 19. 